1. Field of the Invention
The present invention relates to a data transmission system capable of transmitting video data with high bit rates through a wireless transmission path. More specifically, the present invention relates to a data transmission system capable of flexibly coping with a communication failure caused by jamming during data transmission.
2. Description of Related Art
In recent years, DVD/HDD recorders or personal computers with a PVR (Personal Video Recorder) function have been remarkably widespread. Accompanying this tendency, video properties capable of being privately viewed at home are also increasing. Further, it is desired to freely enjoy viewing at any required time or place the contents such as video information stored in DVD/HDD recorders or personal computers. Therefore, there is proposed a technology for enabling the contents to be outputted to client devices such as TV sets or personal computers using a home network. For a technology for transmitting vast quantities of data such as video data or audio data at high speed-in real time, for example, IEEE 1394 is adopted for an interface for connecting digital TV sets or digital videos.
Further, for simplifying the connection between devices, a wireless LAN has been recently used to allow connection of DVD/HDD recorders, personal computers or LCD monitors. For a typical technology using the wireless LAN, IEEE 802.11 standardized in IEEE is adopted, and a technology of using 2.4 GHz band becomes mainstream. The 2.4 GHz band is referred to as an ISM (Industrial, Scientific and Medical) band and is a frequency band used in an industrial, medical or household microwave oven. In Japan, a frequency band from 2.400 to 2.497 GHz is allocated to a wireless LAN.
When using the wireless LAN as a transmission path of digital AV data including video data or audio data, for example, a communication speed from 2 to 30 Mbps per channel is required for video data and a communication speed from 64 kbps to 1.5 Mbps is required for audio data. On the other hand, when using Internet, a communication speed of several Mbps is sufficient only in the case of Web access; however, a communication speed of several tens Mbps is required in the case of content delivery.
Further, there are problems that during the use of wireless LAN, an obstacle gets into a wireless transmission path or interference with communication between other devices occurs (jamming occurs), and as a result, a communication speed falls down or a communication failure occurs. Particularly, a 2.4 GHz frequency band includes much jamming because of being used by many wireless devices. Therefore, various technologies are proposed to solve the above-described problems.
For example, JP 2003-273879A proposes the following reception band management method. That is, when a total band decreases while plural terminals perform data reception, increase in a reception delay time is prevented in a terminal for receiving data (typically, video data for digital television broadcast, or video data or audio data for streaming reproduction) having a short permissible reception delay time, so that generation of reception failure due to the increase in the reception delay time (for example, a phenomenon such that in a digital television, a video frame is dropped or voice communication is interrupted) can be prevented.
JP 2004-266502A proposes the following video transmission system. That is, in a wireless net work in which the transmission band varies, even if the band greatly varies due to movement of a reception terminal, uninterrupted video transmission can be realized. Further, for a conventional technology for preventing video interruption due to band variations, there is disclosed a technology in which in response to a bit rate change request from a reception terminal, a server selects one video stream from among plural video streams with different bit rates to perform transmission.
JP 2004-297628A discloses the following technology. In this technology, characteristics of broadcast and communication are made full use of to establish new video content delivery configurations in which broadcast and communication are blended, and a video stream is delivered and reproduced while switching a first stream obtained through a broadcast wave and a second stream obtained through a communication line such as a network.
JP 2004-320762A discloses the following technology. In this technology, for example, a front end circuit which operates at 2.4 GHz and a front end circuit which operates at 5 GHz are provided in a wireless communication device to correspond to two frequency bands of 2.4 GHz band and 5 GHz band, respectively. By changing communication channels having these frequency bands, the number of channels capable of being simultaneously set within the same area in the wireless LAN system is greatly increased and the possibility of causing a communication link to be interrupted by jamming is reduced. Further, transmission conditions of video data and audio data are reported based on the communication conditions. Therefore, in switching the communication channels, even if display of videos is interrupted by the communication channel change, a user is prevented from feeling uncomfortable.
JP 2004-356695A discloses the following technology. In a receiving device which receives and decodes video data transmitted from a transmitting device, even if capacity of a buffer provided on the prestage of a decoder is reduced, the video data is reproduced without causing underflow or overflow as well as a delay time required to provide contents at the time of operating a remote controller is shortened.
JP 2002-198975A discloses the following communication quality maintenance and management method. That is, in a wireless LAN system, transmission channels are changed to reestablish a link while monitoring a link error.
On the other hand, for a technology for realizing streaming delivery of video data, there is studied a transcoding system in which in response to environments of a communication band of a user, a bit rate of the coded video data (e.g., MPEG 2-TS) is converted and then, data transmission is performed. When using this transcoding system, a flexible response to band variations is allowed in a transmission path with temporal band variations, so that the streaming delivery of high-quality video data can be realized. The above-described document discloses no technology for efficiently avoiding reduction in a communication speed or occurrence of a communication failure in the data transmission using the transcoding system.